This invention relates to a method and apparatus for measuring the radiation dose distribution produced by a radiation source. Specifically, the invention is directed to a method and apparatus for measuring the radiation dose distribution produced by a radiation source which produces radiation in predefined patterns disposed about a central location.
In the field of medicine, radiation is used for diagnostic, therapeutic and palliative treatment of patients. The conventional medical radiation sources used for these treatments include large fixed position machines as well as small, transportable radiation generating probes. The current state of the art treatment systems utilize computers to generate complex treatment plans that require verification to insure proper treatment.
In order to be able to more precisely provide a desired dose and volume of treatment, it is necessary to calibrate the radiation source by accurately measuring the radiation pattern of the source. Since the radiation dose is affected by the density of the irradiated volume, the more dense the material, the more radiation it will absorb, it is desirable to model the volume to be irradiated, and to conduct the calibration procedure on such a model. It is also desirable to perform the treatment plan in such a model, and confirm that the desired radiation dose distribution is being produced.
Conventional radiation treatments systems, such as the LINAC used for medical treatment, utilize a high power remote radiation source and direct a beam of radiation at a target area, such as tumor inside the body of a patient. This type of treatment is referred to as teletherapy because the radiation source is located a predefined distance, approximately one meter, from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation.
Prior to treatment, the teletherapy system is calibrated using an ionization chamber positioned generally along the beam path but distant from the radiation source. The ionization chamber is disposed within a large tank of water, hereinafter referred to as a water phantom. The water in the water phantom approximates the radiation density of living tissue and simulates the body of the patient. During calibration, the ionization chamber is moved within the water phantom and the system accumulates individual radiation dose measurements at predetermined points in the water phantom. The individual dose measurements are utilized to construct a three dimensional dose distribution for the calibration pattern and/or radiation treatment applied.
An alternative treatment system utilizing a point source of radiation is disclosed in U.S. Pat. No. 5,153,900, owned by the assignee of the present application. The system includes a miniaturized, insertable probe capable of producing low power radiation in predefined dose geometries disposed about a predetermined location. This treatment is referred to as brachytherapy because the source is located close to or in some cases within the area receiving treatment. One advantage of brachytherapy is that the radiation is applied primarily to the treatment volume, without significantly affecting the tissue in adjacent volumes.
Prior to treatment, the brachytherapy system must also be calibrated to verify the radiation dose distribution. Conventional water phantoms/ionization chamber calibration techniques are not effective in measuring the dose distribution of the brachytherapy system because there is no way to orient the ionization chamber to keep it pointed at the radiation source for various points within the phantom. It is necessary to orient the ionization chamber in order to obtain high resolution dose measurements. In addition, since brachytherapy is applied only to near-source tissue, the ionization chamber must be very close to the radiation source, typically less than 5 centimeters, and therefore, the movements of the ionization chamber must be controlled to avoid colliding with the probe.
Accordingly, it is an object of the invention to provide a method and apparatus for measuring the dose distribution about a source of radiation.
It is another object of the invention to provide a method and apparatus for measuring the radiation dose distribution about a source at one or more points in the target volume.
It is another object of the invention to provide an apparatus for measuring the radiation dose distribution about a source which can orient the radiation measuring element toward the source.
It is yet another object of the invention to provide an apparatus for measuring the radiation output of a source which is capable of moving the source with respect to the radiation measuring element and which is capable of avoiding a collision between the source and the radiation measuring element.